Distribution of intraperitoneally administered deuterium-labeled water in aquaporin-4-knockout mouse brain after middle cerebral artery occlusion

Abstract

Introduction: As the movement of water in the brain is known to be involved in neural activity and various brain pathologies, the ability to assess water dynamics in the brain will be important for the understanding of brain function and the diagnosis and treatment of brain diseases. Aquaporin-4 (AQP4) is a membrane channel protein that is highly expressed in brain astrocytes and is important for the movement of water molecules in the brain.

Methods: In this study, we investigated the contribution of AQP4 to brain water dynamics by administering deuterium-labeled water (D₂O) intraperitoneally to wild-type and AQP4 knockout (AQP4-ko) mice that had undergone surgical occlusion of the middle cerebral artery (MCA). Water dynamics in the infarct region and on either side of the anterior cerebral artery (ACA) was monitored with proton-density-weighted imaging (PDWI) performed on a 7T animal MRI.

Results: D₂O caused a negative signal change quickly after administration. The AQP4-ko mice showed a delay of the time-to-minimum in both the contralateral and ipsilateral ACA regions compared to wild-type mice. Also, only the AQP4- ko mice showed a delay of the time-to-minimum in the ipsilateral ACA region compared to the contralateral side. In only the wild-type mice, the signal minimum in the ipsilateral ACA region was higher than that in the contralateral ACA region. In the infarct region, the signal attenuation was slower for the AQP4-ko mice in comparison to the wild-type mice.

Discussion: These results suggest that AQP4 loss affects water dynamics in the ACA region not only in the infarct region. Dynamic PDWI after D₂O administration may be a useful tool for showing the effects of AQP4 in vivo.

Keywords: aquaporin-4; brain ischemia; brain water dynamics; deuterium-labeled water; in vivo proton-density-weighted MRI.

FIGURE 1

Schematic defining the timing of deuterium-labeled water (D₂O) administration in comparison to the proton-density-weighted imaging (PDWI). PDWI images were acquired every 20 s for 30 min (90 scans total). Two min after scanning started, D₂O was administered intraperitoneally. T_min indicates the time from administration to the signal minimum.

FIGURE 2

Water dynamics measurements after intraperitoneal administration of deuterium-labeled water (D₂O). (A–C) Representative diffusion-weighted imaging (DWI) images, pre-D₂O baseline proton-density-weighted imaging (PDWI) images and % signal change at 1, 3, and 7 min after D₂O administration for the wild-type mice. (D–F) A similar set of images for the AQP4 knockout (AQP4-ko) mice. (G) Standardized signal changes in each region-of-interest (ROI) for the wild-type mice. (H) Standardized signal changes in each ROI for the AQP4-ko mice. Error bars indicate standard deviations across animals. The Infarct (blue), Ipsi-anterior cerebral artery (ACA) (green), and Contra-ACA (red) ROIs are shown on the DWI image in panel (A).

FIGURE 3

Comparisons of the time-to-min (T_min) and signal minimum (%S_min) in the anterior cerebral artery (ACA) regions-of-interest (ROIs) after deuterium-labeled water (D₂O) administration. (A) T_min for the wild-type (n = 5) and AQP4 knockout (AQP4-ko) (n = 6) mice. (B) %S_min for both mouse genotypes. Results for the same animal are connected by lines. Comparisons between genotypes were made using an unpaired t-test, while the comparison between the ipsi and contralateral sides were made with a paired t-test. The resulting p values are shown at the top of the figure. Note that, although the data from five the wild-type mice are plotted, the data for two of the animals nearly overlap so that it appears that there is only four data points.

FIGURE 4

Signal attenuation in the infarct region. (A) Standardized proton-density-weighted imaging (PDWI) signal in the infarct region of the wild-type (orange) and AQP4 knockout (AQP4-ko) (blue) mice. The slope α is calculated from the signal recorded at 3–5 min after deuterium-labeled water (D₂O) injection. (B) The estimates of α for the two groups were compared using an unpaired t-test and the p value is shown at the top of the figure.